Two Transistor Model of SCR or Thyristor – Circuit Diagram & Analysis

Two transistor model is used to explain the principle of operation of the thyristor in a very simple way. Since a thyristor is a four-layered PNPN device, it can be considered as a combination of two transistors, one transistor as PNP and the other transistor as NPN. A two-transistor model is obtained by separating the two middle layers of the thyristor into two parts as shown in the below figure.

Two Transistor Model of SCR or Thyristor

In this model, base current Ib1 of transistor Q1 is equal to the collector current Ic2 of transistor Q2 and vice-versa. The equivalent circuit of the two transistor analogy is shown below.

Two Transistor Model of SCR or Thyristor

From the above equivalent circuit, the base and collector currents of the transistors Q1 and Q2 can be written as,

Ib1 = Ic2 and Ic1 = Ib2 …(1)

Also, from the above figure, we have, cathode current is the sum of anode current and gate current.

Ik = Ia + Ig …(2)

We know that, collector current Ic is related to emitter current Ie as,

Ic = αIe + Icbo


  • α = Common-base current gain ≃ Ic/Ie
  • Icbo = Leakage current of collector-base.

Hence, in the given equivalent circuit,

Ic1 = α1 Ie1 + Icbo1 and
Ic2 = α2 Ie2 + Icbo2 …(3)

But, Ie1 of Q1 = Ia and Ie2 of Q2 = Ik. Subsituting the values of Ie1, Ie2 in equation 3, we get,

Ic1 = α1 Ia + Icbo1 and
Ic2 = α2 Ik + Icbo2 …(4)

The sum of two collector currents is equal to an anode current. Therefore,

Ia = Ic1 + Ic2 …(5)

Substituting equation 4 in equation 5, we get,

Ia = α1 Ia + Icbo1 + α2 Ik + Icbo2 …(6)

From transistor analysis, we have,

Ie1 = Ib1 + Ic1
Ib1 = Ie1 – Ic1 …(7)

Substituting equation 2 in equation 6, we get,

Two Transistor Model of SCR or Thyristor

If the leakage currents Icbo1 and Icbo2 of transistors Q1 and Q2 are negligibly small, then,

Two Transistor Model of SCR or Thyristor

From the above equation, it can be analyzed that, if α1 + α1 = 1, then anode current Ia becomes infinity and hence thyristor enters into conduction state from non-conduction state. In other words, if Ig reaches a value such that α1 + α2 = 1, then SCR is turned ON and this action is called the Regenerative action.

The regenerative action i.e., the condition α1 + α1 = 1 can be obtained in the following ways,

  • The increase in temperature increases the leakage current by which the required above condition can be obtained to turn on the SCR.
  • The SCR can be also turned ON by increasing α1 and α2. Since α2 depends on (Ia + Ig) and α1 on Ia, the current gain of the transistor Q2 can be increased without affecting voltage Va and current Ia by injecting the current Ig through the gate in the same direction of Ia. Thus the above condition can be obtained.

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